This invention relates to a method of etching a substrate in a chamber on an electrostatic chuck, which defines a gas cooling path at the substrate/chuck interface.
Electrostatic chucks are well known for retaining semi-conductor wafers and other substrates on a chuck utilising electrostatic force. Such chucks depend upon the presence of an electrically conducting layer somewhere within the substrate. For insulating substrates, this layer is often the top film, that is to be etched, which is usually the only conducting layer on the substrate, and so the clamping force can break down at the point at which the conductive layer is etched from the surface of the substrate and becomes discontinuous. This can cause a number of problems including the substrate blowing off the chuck and an inability to perform an over etch step to remove material from facets or features on the substrate.
The present invention consists in a method of etching a substrate in a chamber on an electrostatic chuck which defines the gas cooling path at the substrate/chuck interface including:
i. electrostatically clamping the substrate on the chuck with the gas in the gas path being at a first pressure;
ii. etching the substrate at a first power;
iii. detecting the end point for the etch;
iv. reducing the gas pressure to a second pressure; and
v. over-etching the wafer at a second power, which is lower than the first power.
It will be observed that the Applicant has realised that by reducing the gas pressure one can reach a point at which the substrate simply floats on the gas; the actual pressure therefore being essentially determined by the mass of the substrate. Because the over-etching is then carried out at a reduced power, the reduced gas flow can still provide sufficient cooling. As the over-etch is only required for a very limited period for the removal of a small amount of residual material, the reduced etching rate is acceptable for throughput purposes.
It is particularly preferred that the substrate is laterally restrained at least during steps iv and/or v and conveniently this can be done by providing a circumjacent wall, which may form part of a uniformity ring. As the wall needs to confine the wafer to its processing position, the clearance between the substrate edge and the wall must be small and it is therefore preferred that the uniformity ring has an enlarged lead in mouth.
Conveniently the cooling gas may be helium and for a typical semi-conductor wafer, the first pressure will be in the range of 6-9.5 torr, whilst the second pressure will be in the range 1-2 torr. However, as has been indicated above, the second pressure in particular is to a great extent determined by the mass of the substrate.
The gas path may, at least in part, be defined by radial and/or coaxial grooves in the chuck surface, in which case it is preferred that the grooves are wider than they are deep. This is particularly the case where the electrostatic chuck has a thick dielectric layer and is being operated at voltages in the region of a few thousand volts. The wide grooves are easier to post form and in the above construction, the loss of clamping over the width of the groove is not a serious problem. Thus the grooves may be between 1 and 5 mm wide and of the order of 200 xcexcm deep.
It is particularly preferred that these wide and shallow grooves are used in a so called xe2x80x9cthick dielectricxe2x80x9d electrostatic chuck such as described in our U.S. patent application Ser. No. 09/150,669, now U.S. Pat. No. 6,256,186, the contents of which are incorporated hereby incorporated by reference. Such thick chucks allow significant voltages to be used and the lack of clamping over the width of the groove is less significant. It has also been found that such chucks are particularly effective at clamping III/V compound wafers, which are often processed on sapphire carriers. The width of the grooves enhances cooling at the low pressures used when the wafer becomes unclamped.
Although the invention has been defined above it is to be understood it includes any inventive combination of the features set out above or in the following description.